Ultrasonic imaging is used as a diagnostic tool to aid in therapeutic procedures. Ultrasonic energy is produced by generating and receiving sound waves with an ultrasonic scanner. Contrast agents which are echogenic are preferentially used to create ultrasonic energy in an area of interest. In ultrasound imaging, videotape images obtained following contrast injection are digitized, allowing the gray scale to be quantified from 1 to 225 gray scale units for 30 cardiac cycles. The contrast intensity is plotted on the vertical axis against time on the horizontal axis. The peak videointensity (corrected for baseline intensity) is determined as the highest point on the time intensity curve. For a discussion of contrast echographic instrumentation, see, for example, De Jong N, "Acoustic properties of ultrasound contrast agents", CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG (1993), pages 120 et seq.
Contrast echocardiography has been used to delineate intracardiac structures, assess valvular competence, and demonstrate intracardiac shunts. Myocardial contrast echocardiography (MCE) has been used to measure coronary blood flow reserve in humans. MCE has been found to be a safe and useful technique for evaluating relative changes in myocardial perfusion and delineating areas at risk.
A multiplicity of potential ultrasonic imaging agents has been reported for contrast echocardiography. It is believed that no such agent routinely attains visually discernible myocardial uptake following peripheral intravenous injection. Although there have been many reports of transpulmonary transmission of ultrasound contrast agents following intravenous injection and despite the fact that myocardial opacification on echocardiogram can be produced by left sided injection of such contrast agents, visualization of myocardial contrast has not been achieved by intravenous administration of sonicated microbubbles.
Most recently, sonicated albumin and sonicated dextrose/albumin have been shown to produce variable degrees of left ventricular chamber ultrasound contrast following intravenous injection. (See Villanueva et al. Circulation 85:1557-1564, 1992; Lin et al. Int J Card Imaging 8:53-6, 1992; Feinstein et al. J Am Coll Cardiol 16:316-224, 1990; Keller et al. Am Heart J 114:570-575, 1987; and Shapiro et al. J Am Coll Cardiol 16:1603-1607, 1990). The microbubbles of these contrast agents are small (4-6 microns) and are capable of swift transpulmonary passage. However, visually discernible myocardial uptake of such microbubbles following peripheral intravenous injection has not been possible because of the rapid diffusion of blood soluble oxygen and nitrogen inside the microbubble into the blood which consequently loses its ultrasound reflective properties (e.g., see Porter et al. J Am Soc Echocard Supplement 7:S1, May 1994, and Weyman AE: Principles and Practice of Echocardiography, Malvern, Pa.: Lea & Febiger, 1994; pp.302-26.)
An important objective of this invention is to provide a contrast agent and methods for its production and use wherein microbubble survival and subsequent myocardial ultrasound contrast is improved sufficiently to make possible visually discernible myocardial uptake of such microbubbles following non-invasive peripheral intravenous injection. This and other objectives of this invention will become apparent in the following discussion.